I need to replace my main halyard on Parker 275 but see that the original, per the manual, was 8mm terylene whereas chandlery recommendation for this sail area is 10mm if using standard double braid polyester. I note that the manual for the Parker 31 states the halyard size there is 10mm. Is it advisable to upgrade to 10mm or would this be a problem with the size of the sheave at the masthead and the fittings at the base of the mast? Or would it be better to stick to 8mm and use one of the newer materials that are stronger?

Staying with 8mm is likely to make raising and lowering the sail easier.
An good option would be 8mm dyneema; horribly expensive but very low stretch which makes controlling the sail shape easier.

Martin
P335/50

Hi, I replaced my main halyard with 10mm braided polyester a while ago. The main reason was that the 8mm was slipping in the clutch (and yes I had to replace the clutch in the end!), but it was also reassuring to have something stronger. The main does not rattle down, so there may be a bit more friction, but it comes down with very little encouragement, .
I guess I am reassuring you that 10mm braided polyester is a practical alternative.
Bob S